1. Field of the Invention
The present invention relates to a wavelength division multiplexing coupler used in wavelength division multiplexing communication in an optical communication field, particularly relates to a wavelength division multiplexing optical coupler using an optical filter.
2. Related Art
There is PON (Passive Optical Network) as one of network systems used for FTTx (Fiber To The x, x=H: Home, P: Premise, C: Curb or the like) for introducing optical communication to a so-to-speak access system between a subscriber and a station. There are used fluxes of light having different wavelengths for upstream data from a subscriber to a station and downstream data from a station to a subscriber therefor. Further, there is also a case of multiplexing to use an analog signal of an image or the like having a further different wavelength. For example, there are used 1310 nm band for Upstream Data, 1490 nm band for Downstream Data and 1550 nm band for an image signal. Therefore, there is needed a wavelength division multiplexing optical coupler for multiplexing/demultiplexing the signals having the respective wavelengths in OLT (Optical Line Termination) or ONU (Optical Network Unit) provided on a station side and on a subscriber side.
In a background art, the wavelength division multiplexing optical coupler used for the above-described object is realized by a constitution as shown by FIG. 6. In order to multiplexing/demultiplexing three wavelengths λ1, λ2, λ3, there are used three pieces of optical couplers 201, 202, 203 respectively having three kinds of band pass filters constituting transmission wavelength bands by three different wavelength bands which are connected in Cascade connection (refer to, for example, JP-A-54-17044). The optical coupler is referred to as 3 port coupler and when incident light multiplexed with two wavelengths is made to be incident from an optical fiber 223 constituting a common port (light incident port), fluxes of emitting light of two wavelength regions can respectively be outputted from optical fibers 224, 225 constituting two light emitting ports. By connecting the 3 port couplers in cascade connection, 3 or more wavelengths can be separated. Further, by increasing a number of the cascade connections, the function can be expanded to multiplexing/demultiplexing of 4 or more of wavelength bands. Further, the final optical coupler 203 can also be omitted.
As an inner constitution of the 3 port coupler, it is known to insert a band pass filter between a two-core collimator comprising two pieces of optical fibers and collimator lenses and a single core collimator comprising one piece of an optical fiber and a collimator lens, a type of inserting a band pass filter to a dividing portion of an optical wave guide divided in a Y-like shape or the like. There may be used an optical coupler having 2 kinds of edge filters (there is long wavelength transmitting type or a short wavelength transmitting type) having edge portions (wavelength edges) of transmitting wavelength bands at middles of three wavelength bands in place of a band pass filter.
A cross talk hampering amount (isolation) indicating to what degree is an amount of mixing light having a wavelength other than a predetermined wavelength band to each port of the optical coupler of the background art is normally about 12 dB owing to an influence of remaining reflection which is reflection which is not related to a wavelength characteristic of a filter in the case of a port utilizing reflected light of each filter and achieves at most about 18 db as a limit even when transmission ripples can pertinently be adjusted. Therefore, it is difficult to achieve high isolation (20 dB or higher) for all the channels.
Further, in order to connect two pieces (or three pieces) of couplers in cascade connection, it is necessary to lead around optical fibers and in that case, in order to prevent optical loss from being brought about, a winding radius of the optical fiber cannot be reduced to be equal to or smaller than a constant degree. Since a cabinet containing the optical fibers is needed, the coupler cannot be constituted by a shape of a small-sized tube as in the normal 3 port coupler but the coupler becomes large-sized.
Further, a number of parts is increased by an amount of a number of cascade connections of the 3 port couplers and cost is increased by that amount.
There is known an optical coupler overlappingly inserting a plurality of edge filters having different edge wavelengths in place of 1 sheet of the above-described optical filter as a measure for resolving the above-described problem (refer to, for example, U.S. Pat. No. 4,474,424). This is an optical coupler for coupling fluxes of light having different wavelengths to different optical fibers by changing directions of fluxes of reflected light from respective filters by respectively changing angles of respective optical filters. Thereby, separation of three wavelengths or more can be carried out by one optical coupler without connecting 3 port couplers in cascade connection and the above-described problem of large-sized formation or an increase in the number of parts can be resolved.
However, according to the above-described constitution, basically, fluxes of reflected light by the respective optical filters are utilized and therefore, the problem that only isolation of about 12 dB can be achieved normally owing to remaining reflection cannot be resolved.